At EQI.org we define
emotional support as helping
someone meet their emotional needs. These needs include
the need to feel accepted, cared about, believed in,
important, listened to, safe, understood, valued. Here
are more emotional needs.

According to The
American Institute of Stress,
emotional support seems most effective when we can share
our feelings or burdens with others who are experiencing
or have experienced similar problems. (Source)

I would say it is also
most effective when there is a large amount of empathy
and understanding, and an absence of invalidation
and judgement.